Air quality indices (AQI) are commonly used to indicate the level of severity of air pollution to the public. A number of methods were developed in the past by various researchers/environmental agencies for the calculation of AQI, but there is no universally accepted method, appropriate for all situations. An updated review of the major air quality indices developed worldwide is presented in this paper. These methods differentiate mainly in the number of pollutants included, its sampling period and air quality classes and breakpoints. When applying different AQI to a common case study, important differences are found in terms of the classification of the quality of the air. The purposes of this research are to identify weaknesses of the current AQI and to discuss possible changes and updates with Portugal as case study. A survey, with 10 questions about the calculation and use of the AQI and its dissemination to public, was delivered to the five regional environmental agencies in Portugal and, based on results, modifications to the current AQI are proposed. Two main changes-inclusion of PM2.5 and specific urban/industrial AQI-were tested, comparing the current and the proposed AQI along the 2014 year. It is observed that a significant difference exists when specific urban and industrial sites are considered when calculating the AQI. On the other hand, and contrarily to other regional studies, the results show that the inclusion of fine suspended particulate (PM2.5) does not impact the final AQI value.

Portuguese Agency for the Environment; Regional Coordination and Development Commission (CCDRs); FEDER through the COMPETE Programme; national funds from FCT-Science and Technology Portuguese Foundation [SFRH/BD/87468/2012]; CESAM [UID/AMB/50017]; FCT/MEC through national funds; FEDER, within the PT Partnership Agreement and Compete

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The authors wish to thank the Portuguese Agency for the Environment and the Regional Coordination and Development Commission (CCDRs) for their support. Also, the authors would like to acknowledge the financial support of FEDER through the COMPETE Programme and the national funds from FCT-Science and Technology Portuguese Foundation for the Ph.D. grant of C. Gama (SFRH/BD/87468/2012). Thanks are also due for the financial support to CESAM (UID/AMB/50017), to FCT/MEC through national funds, and the co-funding by the FEDER, within the PT2020 Partnership Agreement and Compete 2020.